1. Field of the Invention
The present invention relates to a rotation angle detecting device capable of magnetically detecting, for instance, a rotation angle of a gear-shaped magnetic rotation member.
2. Description of the Related Art
Referring now to a drawing, a conventional rotation angle detecting device will be described. FIG. 4(a) to FIG. 4(c) are diagrams for illustratively showing a magnetic circuit of a conventional rotation angle detecting device. FIG. 5 represents a signal processing circuit employed in the conventional rotation angle detecting device.
In FIG. 4, reference numeral 1 shows a rotation shaft corresponding to a crank shaft of a vehicle engine, reference numeral 2 represents a magnetic rotation member, reference numeral 3 indicates a magnet, reference numeral 4 denotes a chip, and reference numeral 5 shows a magnetic detecting element (will be referred to as an xe2x80x9cMR elementxe2x80x9d hereinafter).
In FIG. 5, reference numeral 10 shows a bridge circuit, reference numeral 20 indicates a differential amplifying circuit, reference numeral 30 represents an AC coupling circuit, reference numeral 40 indicates another differential amplifying circuit, reference numeral 50 shows a comparing circuit, reference numeral 60 indicates an output circuit, and also reference numeral 70 is an initiating circuit.
As illustrated in FIG. 4, the conventional rotation angle detecting circuit contains the magnet 3 having a rectangular solid shape, the chip 4, and the magnetic detecting element 5. This chip 4 is mounted on an upper surface of this magnet 3, and a semiconductor integrated circuit (namely, signal processing circuit) is built in this chip 4.
Since the magnetic rotation member 2 having the gear shape is rotated which is provided in the vicinity of the above-explained rotation angle detecting device, a concave portion and a convex portion of the magnetic rotation member 2 are alternatively positioned in the vicinity of the magnetic detecting element 5. As a result, a magnetic field, which is applied from the magnet 3 to the magnetic detecting element 5, is changed. This change in the magnetic fields may be detected as a change in voltages by the magnetic detecting element 5.
This change in the voltages is outputted as a pulsatory electric signal through the differential amplifying circuit 20, the AC coupling circuit 30, the differential amplifying circuit 40, the comparing circuit 50, and the output circuit 60, which are provided in the chip 4, to an external circuit. This pulsatory electric signal is supplied to a computer unit (not shown), so that a rotation angle of the magnetic rotation member 2 may be detected.
Generally speaking, as the magnetic detecting element 5, either a magnetic resistance element (will be referred to as a xe2x80x9cMR elementxe2x80x9d hereinafter) or a giant magnetic resistance element (will be referred to as a xe2x80x9cGMR elementxe2x80x9d hereinafter) is employed. However, since the operations of these MR element and GMR element are substantially identical to each other, operations of the rotation detecting device in the case that the MR element is employed will be explained in detail.
An MR element (magnetic resistance element) corresponds to such an element whose resistance value is changed based upon an angle defined between a magnetization direction of a thin film of a ferromagnetic material (for example, Nixe2x80x94Fe, Nixe2x80x94Co etc.) and a current direction thereof. The resistance value of this MR element becomes minimum when the current direction is intersected with the magnetization direction at a right angle, whereas the resistance value of the MR element becomes maximum in the case that the current direction is intersected with the magnetization direction at an angle of 0 degree. In other words, when both the current direction and the magnetization direction are intersected along the same direction, or completely opposite directions, this resistance value becomes maximum. This change in the resistance values will be referred to as an MR change rate. Generally speaking, the MR change rate of Nixe2x80x94Fe is equal to 2 to 3%, and the MR change rate of Nixe2x80x94Co is equal to 5 to 6%.
Since the magnetic rotation member 2 is rotated, a magnetic field applied to the MR element 5 is changed, so that the magnetic resistance value of this MR element 5 is changed. Accordingly, in order to detect the change in the magnetic fields, as indicated in FIG. 5, the bridge circuit 10 is constituted by employing the MR element 5. While this bridge circuit 10 is connected to a power supply capable of supplying a constant voltage and a constant current, the resistance value change of the MR element 5 is converted into a voltage change, and then, a change in magnetic fields which is exerted to this MR element 5 may be detected.
The conventional rotation angle detecting device is arranged by employing the bridge circuit 10, the differential amplifying circuit 20, the AC coupling circuit 30, the differential amplifying circuit 40, the comparing circuit 50, the initiating circuit 70, and the output circuit 60. The bridge circuit 10 is constructed of the MR element 5. The differential amplifying circuit 20 amplifies the output signal of this bridge circuit 10. The AC coupling circuit 30 removes a DC component from the output signal of the differential amplifying circuit 20. The differential amplifying circuit 40 amplifies the output signal of this AC coupling circuit 30. The comparing circuit 50 compares the output signal of this differential amplifying circuit 40 with a reference value to output either a signal of xe2x80x9c0xe2x80x9d or a signal of xe2x80x9c1xe2x80x9d. The initiating circuit 70 sets the output signal of this comparing circuit 50 to a preselected level. The output circuit 60 is operated in a switching mode in response to the output signal of this comparing circuit 50.
The bridge circuit 10 contains a resistor 11 and the above-described MR element 5. The resistor 11 is connected to a power supply terminal VCC; the MR element 5 is connected to the ground; and the other respective terminals of the resistor 11 and the MR element 5 are connected to a junction point xe2x80x9cA.xe2x80x9d
Then, the connection point xe2x80x9cAxe2x80x9d of the bridge circuit 10 is connected via a resistor 21 to an inverting input terminal of an operational amplifier 20 employed in the differential amplifying circuit 20. Also, a non-inverting input terminal of this operational amplifier 22 is connected via a resistor 27 to a voltage dividing circuit which constitutes a reference power supply, and is further connected via a resistor 24 to the ground. The output terminal of the operational amplifier 22 is connected via the resistor 23 to the own inverting input terminal, and also is connected to a capacitor 31 employed in the AC coupling circuit 30.
The AC coupling circuit 30 is arranged by one capacitor 31 and a resistor 34. A junction point between the capacitor 31 and the resistor 34 is connected via the resistor 41 to an inverting input terminal of an operational amplifier 42 employed in the differential amplifying circuit 40. Also, another terminal of the resistor 34 is connected to a voltage dividing circuit which constitutes a reference power supply.
Also, a non-inverting input terminal of the operational amplifier 42 provided in the differential amplifying circuit 40 is connected via a resistor 47 to the voltage dividing circuit which constitutes the reference power supply, and further, is connected via a resistor 44 to the ground. An output terminal of this operational amplifier 42 employed in the differential amplifying circuit 40 is connected via a resistor 43 to the own inverting input terminal, and also is connected to an inverting input terminal of an operational amplifier 51 employed in the comparing circuit 50.
Both a non-inverting input terminal and an output terminal of an operational amplifier 77 employed in the initiating circuit 70 are connected to the output terminal (namely, junction point between capacitor 31 and resistor 34) of the AC coupling circuit 30, and the non-inverting input terminal of this operational amplifier 77 is connected to a reference voltage unit (voltage dividing circuit) of the AC coupling circuit 30.
A non-inverting input terminal of an operational amplifier 51 employed in the comparing circuit 50 is connected to the voltage dividing circuit, and an output terminal of this operational amplifier 51 is connected to a base of a transistor 61 provided in the output circuit 60.
A collector of the transistor 61 provided in the output circuit 60 is connected to a transistor 62 and also a base of a transistor 56 employed in the comparing circuit 50, and also is connected via a resistor 63 to the power supply terminal VCC. An emitter of this transistor 61 is connected to the ground. A collector of a transistor 62 is connected to an output terminal 65, and also connected via a resistor 64 to the power supply VCC, and further, an emitter of this transistor 62 is connected to the ground.
In addition, a collector of a transistor 56 employed in the comparing circuit 50 is connected to a voltage dividing circuit which constitutes a reference power supply of the comparing circuit 50, and an emitter of this transistor 56 is connected to the ground.
FIG. 6 is a timing chart for indicating a waveform processing operation obtained while the magnetic rotation member is rotated in the above-explained conventional rotation angle detecting device.
Since the magnetic rotation member 2 is rotated, a magnetic field change is applied to the MR element 5, so that such an output as shown in FIG. 6(a) is obtained from the output side of the differential amplifying circuit 20. This output signal corresponds to the concave/convex portions of the magnetic rotation member 2.
The output signal of this differential amplifying circuit 20 is supplied to the AC coupling circuit 30, and then, a DC component is removed from this supplied signal by this AC coupling circuit 30. At the same time, a reference voltage (for example, (xc2xd)xc3x97VCC) is applied as a DC component. Then, the output signal of this AC coupling circuit 30 is furthermore amplified by the differential amplifying circuit 40, and then, the amplified signal is supplied to the comparing circuit 50.
As indicated in FIG. 6(a), the signal (input signal of comparing circuit) which is supplied to the comparing circuit 50 is compared with a reference value equal to a comparison level so as to be converted into either a signal of xe2x80x9c0xe2x80x9d or a signal xe2x80x9c1.xe2x80x9d
This comparison signal is furthermore waveform-shaped by the output circuit 60. As a result, as represented in FIG. 6(b), such an output signal having xe2x80x9c0xe2x80x9d (LOW), or having xe2x80x9c1xe2x80x9d (HIGH) is obtained from the output terminal 65 of the output circuit 60. These signals have a steep rising edge and a steep falling edge.
In the above-described conventional rotation angle detecting device, since the operational amplifier 77 is used in the initiating circuit 70, there is a problem in that the circuit scale of this rotation angle detecting device is increased, while the initiating circuit 70 sets the signal level of the comparing circuit 50 to a preselected level when the power supply is turned ON.
The present invention has been made to solve the above-explained problem, and therefore, has an object to provide such a rotation angle detecting device capable of instantaneously stabilizing a signal having a desired level and outputting the stable signal when the power supply is turned ON, and also capable of reducing a circuit scale thereof.
To achieve the above-explained object, a rotation angle detecting device according to the present invention is featured by such a rotation angle detecting device comprising: a bridge circuit in which while a magnetic field applied to a magnetic detecting element is changed by rotating a magnetic rotation member mounted on a predetermined rotation shaft, the magnetic field change is converted into a voltage change; a first differential amplifying circuit for amplifying the signal outputted from the bridge circuit; an AC (alternating current) coupling circuit for removing a DC component from the output signal of the first differential amplifying circuit; a second differential amplifying circuit for amplifying the signal output from the AC coupling circuit; a comparing circuit for comparing the signal outputted from the second differential amplifying circuit with a predetermined reference value to thereby output any one of a signal of xe2x80x9c0xe2x80x9d and a signal of xe2x80x9c1xe2x80x9d; an output circuit for waveform-shaping the signal having xe2x80x9c0xe2x80x9d, or xe2x80x9c1xe2x80x9d outputted from the comparing circuit; and an initiating circuit including a first transistor, a second transistor, a third transistor, and a differentiating circuit for driving the first to third transistors; in which both the first transistor and the second transistor are connected to the AC coupling circuit so as to converge the output signal of the AC coupling circuit into a reference voltage immediately after a power supply is turned ON; and in which the third transistor is connected to the comparing circuit so as to also converge the input signal of the comparing circuit into an AC coupling reference level.
Also, the rotation angle detecting device according to the present invention is featured by that both a collector of the first transistor and an emitter of the second transistor are connected to an output terminal of the AC coupling circuit; both an emitter of the first transistor and a collector of the second transistor are connected to a reference voltage unit of the AC coupling circuit; a collector of the third transistor is connected to the output terminal of the comparing circuit; and the differentiating circuit is constituted by a resistor and a capacitor, and is connected to the bases of the first, second, and third transistors so as to drive the first, second, and third transistors only for a give period of time which is determined based upon both the resistance value of the resistor and the capacitance value of the capacitor.